Process for cracking an olefin-rich hydrocarbon feedstock

ABSTRACT

A process for cracking an olefin containing hydrocarbon feedstock which is selective towards light olefins in the effluent, the process comprising passing a hydrocarbon feedstock containing one or more olefins through a moving bed reactor containing a crystalline silicate catalyst selected from an MFI-type crystalline silicate having a silicontalurniniumn atomic ratio of a least 180 and an MEL-type crystalline silicate having a silicon/aluminium atomic ration of from 150 to 800 which has been subjected to a steaming step, at an inlet temperature of from 500 to 600° C., at an olefin partial pressure of from 0.1 to 2 bars and the feedstock being passed over the catalyst at an LHSV of from 5 to 30 h −1  to produce an effluent with an olefin content of lower molecular weight than that of the feedstock, intermittently removing a first fraction of the catalyst from the moving bed reactor, regenerating the first fraction of the catalyst in a regenerator and intermittently feeding into the moving bed reactor a second fraction of the catalyst which has been regenerated in the regenerator, the catalyst regeneration rate being controlled whereby the propylene purity is maintained constant at a value corresponding to the average value observed in a fixed bed reactor using the same feedstock:, catalyst and cracking conditions, for example at least 94 wt %.

The present invention relates to a process for cracking an olefin-richhydrocarbon feedstock which is selective towards light olefins in theeffluent. In particular, olefinic feedstocks from refineries orpetrochemical plants can be converted selectively so as to redistributethe olefin content of the feedstock in the resultant effluent.

It is known in the art to use zeolites to convert long chain paraffinsinto lighter products, for example in the catalytic de-waxing ofpetroleum feedstocks. While it is not the objective of de-waxing, atleast parts of the paraffinic hydrocarbons are converted into olefins.It is known in such processes to use crystalline silicates for exampleof the MFI or MEL type, the three-letter designations “MFI” and “MEL”each representing a particular crystalline silicate structure type asestablished by the Structure Commission of the International ZeoliteAssociation. Examples of a crystalline silicate of the MFI type are thesynthetic zeolite ZSM-5 and silicalite and other MFI type crystallinesilicates are known in the art. An example of a crystalline silicate ofthe MEL type is the synthetic zeolite ZSM-11.

EP-A-0305720 discloses the production of gaseous olefins by catalyticconversion of hydrocarbons. EP-B-0347003 discloses a process for theconversion of a hydrocarbonaceous feedstock into light olefins.WO-A-90/11338 discloses a process for the conversion of C₂-C₁₂paraffinic hydrocarbons to petrochemical feedstocks, in particular to C₂to C₄ olefins. U.S. Pat. No. 5,043,522 and EP-A-0395345 disclose theproduction of olefins from paraffins having four or more carbon atoms.EP-A-0511013 discloses the production of olefins from hydrocarbons usinga steam activated catalyst containing phosphorous and H-ZSM-5. U.S. Pat.No. 4,810,356 discloses a process for the treatment of gas oils byde-waxing over a silicalite catalyst. GB-A-2156845 discloses theproduction of isobutylene from propylene or a mixture of hydrocarbonscontaining propylene. GB-A-2159833 discloses the production of aisobutylene by the catalytic cracking of light distillates.

It is known in the art that for the crystalline silicates exemplifiedabove, long chain olefins tend to crack at a much higher rate than thecorresponding long chain paraffins.

It is further known that when crystalline silicates are employed ascatalysts for the conversion of paraffins into olefins, such conversionis not stable against time. The conversion rate decreases as the time onstream increases, which is due to formation of coke (carbon) which isdeposited on the catalyst.

These known processes are employed to crack heavy paraffinic moleculesinto lighter molecules. However, when it is desired to producepropylene, not only are the yields low but also the stability of thecrystalline silicate catalyst is low. For example, in an FCC unit atypical propylene output is 3.5 wt %. The propylene output may beincreased to up to about 7-8 wt % propylene from the FCC unit byintroducing the known ZSM-5 catalyst into the FCC unit to “squeeze” outmore propylene from the incoming hydrocarbon feedstock being cracked.Not only is this increase in yield quite small, but also the ZSM-5catalyst has low stability in the FCC unit.

There is an increasing demand for propylene in particular for themanufacture of polypropylene.

The petrochemical industry is presently facing a major squeeze inpropylene availability as a result of the growth in propylenederivatives, especially polypropylene. Traditional methods to increasepropylene production are not entirely satisfactory. For example,additional naphtha steam cracking units which produce about twice asmuch ethylene as propylene are an expensive way to yield propylene sincethe feedstock is valuable and the capital investment is very high.Naphtha is in competition as a feedstock for steam crackers because itis a base for the production of gasoline in the refinery. Propanedehydrogenation gives a high yield of propylene but the feedstock(propane) is only cost effective during limited periods of the year,making the process expensive and limiting the production of propylene.Propylene is obtained from FCC units but at a relatively low yield andincreasing the yield has proven to be expensive and limited. Yet anotherroute known as metathesis or disproportionation enables the productionof propylene from ethylene and butene. Often, combined with a steamcracker, this technology is expensive since it uses ethylene as afeedstock which is at least as valuable as propylene.

Thus there is a need for a high yield propylene production method whichcan readily be integrated into a refinery or petrochemical plant, takingadvantage of feedstocks that are less valuable for the market place(having few alternatives on the market).

EP-A-0921179 in the name of Fina Research S.A. discloses the productionof olefins by catalytic cracking of an olefin-rich hydrocarbon feedstockwhich is selective towards light olefins in the effluent. While it isdisclosed in that document that the catalyst has good stability, i.e.high activity over time, and a stable olefin conversion and a stableproduct distribution over time, nevertheless the catalyst stabilitystill requires improvement, particularly when higher inlet temperaturewithin the broad range disclosed (500 to 600° C.) are employed inconjunction with a single reactor. That specification exemplifies theuse of a fixed bed reactor, although it is disclosed that a moving bedreactor, of the continuous catalytic reforming type, or a fluidised bedreactor may be employed for the olefin-cracking process.

During hydrocarbon conversion reactions, a carbonaceous material, i.e.,coke, can be formed and deposited on a catalyst thereby causing it tolose activity. The deposited carbonaceous material on the catalystaffects the amount of active catalyst centres on the catalyst andthereby influences the extent of the hydrocarbon conversion reaction,and hence the conversion to desired products and by-products. Thepresence of carbonaceous material on the catalyst results in a changingproduct distribution that affects the downstream fractionation sectionand the recycle rate of unconverted hydrocarbon feed. For mosthydrocarbons conversion process the loss of activity can be compensatedby increasing the reaction temperature up to a value where undesirableside reactions become important or up to a value which becomesimpracticable.

Thus, it is further known in hydrocarbon conversion processes partiallyto regenerate a catalyst using a moving bed reactor. U.S. Pat. No.3,838,039 discloses a method of operating a continuous hydrocarbonprocess employing catalyst particles in which catalyst activity ismaintained by continuous regeneration. EF-A-0273592 discloses a processfor continuous de-waxing of hydrocarbon oils including reactivation ofpartially spent catalyst. U.S. Pat. No. 5,157,181 discloses a moving bedhydrocarbon conversion process incorporating partial regeneration of aco-catalyst. U.S. Pat. No. 3,978,150 discloses a continuous paraffindehydrogenation process incorporating partial catalyst regeneration.U.S. Pat. No. 5,336,829 discloses a continuous process for thedehydrogenation of paraffinic to olefinic hydrocarbons incorporatingcatalyst regeneration. U.S. Pat. No. 5,370,786 discloses a method ofoperating a continuous conversion process employing solid catalystparticles in which the catalyst may be regenerated. U.S. Pat. No.4,973,780 discloses the alkylation of benzene in a moving bedincorporating partial catalyst regeneration. U.S. Pat. No. 5,849,976discloses a moving bed solid catalyst hydrocarbon alkylation processincorporating partial catalyst regeneration. U.S. Pat. No. 5,087,783discloses the transalkylation of benzene in a moving bed, incorporatingpartial catalyst reactivation. EP-A-0385538 discloses a process for theconversion of a straight-run hydrocarbonaceous feedstock containinghydrocarbons having such a boiling range that an amount thereof boils ata temperature of at least 330° C., such as a gas oil, in a moving bedreactor which may incorporate catalyst regeneration of the zeolitecatalyst. EP-A-0167325 discloses a process for changeover of a movingbed catalytic cracking unit's catalyst inventory from conventionalcatalyst to ZSM-5 containing catalyst, the feedstock comprising an oilchangestock for example a blend of crude oils or a gas oil fraction.U.S. Pat. No. 4,927,526 discloses a process for catalytically crackinghydrocarbon feedstock in a cracking unit to a product comprisinggasoline with an increased octane number in the presence of a crackingcatalyst, under cracking conditions. The process may employ moving bedcatalytic cracking, with changeover of the catalyst inventory.

While the use of a moving bed employing partial catalyst regeneration orreactivation has been known in the art for some time, this, to theapplicant's knowledge, has not been disclosed heretofore for use in anolefin-cracking process.

The olefin-cracking process as disclosed in EP-A-0921179 may be carriedout at high reaction temperature close to the temperature of thermalcracking of hydrocarbon molecules. However, raising the reactiontemperature in order to compensate the loss of catalytic activity in theolefin-cracking process is limited, as it will favour undesirable sidereactions that are not the result of the presence of the catalyst.Moreover, the surface temperatures required to heat up the feed mixturein for instance a fire heater can become so high that thermal crackingof the feed starts.

When the olefin-cracking process of EP-A-0921179 is applied in a fixedbed reactor, it is observed that at the start of the catalytic cyclesmall amounts of less desired products like propane are produced. Thisresults in a lower propylene purity of the C3 fraction. Moreover, theethylene production rate is higher at the start of the catalytic cyclethan after some time. The amount of the less desired product, propane,decreases during the operation and also the ethylene product decreases.During an important period of time the propylene yield remains fairlyconstant while those of propane and ethylene progressively decreases.These variations during the use of the catalyst in a fixed bed reactorare the result of a changing performance of the catalyst caused by thecarbonaceous material laydown.

It is an object of the present invention to provide a process for usingthe less valuable olefins present in refinery and petrochemical plantsas a feedstock for a process which, in contrast to the prior artprocesses referred to above, catalytically converts olefins into lighterolefins, and in particular propylene, and which process has improvedcatalyst stability.

It is another object of the invention to provide a process for producingolefins having a high propylene yield and purity, most particularlysubstantially constantly over the whole time of the process.

The present invention provides a process for cracking anolefin-containing hydrocarbon feedstock which is selective towards lightolefins in the effluent, the process comprising passing a hydrocarbonfeedstock containing one or more olefins through a moving bed reactorcontaining a crystalline silicate catalyst selected from an MFI-typecrystalline silicate having a silicon/aluminium atomic ratio of at least180 and an MEL-type crystalline silicate having a silicon/aluminiumatomic ratio of from 150 to 800 which has been subjected to a steamingstep, at an inlet temperature of from 500 to 600° C., at an olefinpartial pressure of from 0.1 to 2 bars and the feedstock being passedover the catalyst at an LHSV of from 5 to 30 h⁻¹ to produce an effluentwith an olefin content of lower molecular weight than that of thefeedstock, intermittently removing a first fraction of the catalyst fromthe moving bed reactor, regenerating the first fraction of the catalystin a regenerator and intermittently feeding into the moving bed reactora second fraction of the catalyst which has been regenerated in theregenerator, the catalyst regeneration rate being controlled whereby thepropylene purity is maintained constant at a value corresponding to theaverage value observed in a fixed bed reactor using the same feedstock,catalyst and cracking conditions, for example at least 94 wt %.

Preferably, the catalyst regeneration rate is controlled whereby theethylene yield on an olefin basis is less than 10 wt %.

The present invention further provides a process for cracking anolefin-containing hydrocarbon feedstock which is selective towards lightolefins in the effluent, the process comprising passing a hydrocarbonfeedstock containing one or more olefins through a moving bed reactorcontaining a crystalline silicate catalyst selected from an MFI-typecrystalline silicate having a silicon/aluminium atomic ratio of at least180 and an MEL-type crystalline silicate having a silicon/aluminiumatomic ratio of from 150 to 800 which has been subjected to a steamingstep, at an inlet temperature of from 500 to 600° C., at an olefinpartial pressure of from 0.1 to 2 bars and the feedstock being passedover the catalyst at an LHSV of from 5 to 30 h⁻¹ to produce an effluentwith an olefin content of lower molecular weight than that of thefeedstock, intermittently removing a first fraction of the catalyst fromthe moving bed reactor, regenerating the first fraction of the catalystin a regenerator and intermittently feeding into the moving bed reactora second fraction of the catalyst which has been regenerated in theregenerator, the catalyst regeneration rate being controlled whereby allof the catalyst in the moving bed reactor is regenerated in a period offrom 20 to 240 hours.

Preferably, the regeneration rate is controlled whereby the propylenepurity is maintained constant at a value corresponding to the averagevalue obtained in a fixed bed reactor using the same feedstock, catalystand cracking conditions, for example at least 94 wt %.

More preferably, the regeneration rate is controlled whereby theethylene yield on an olefin basis is less than 10 wt %.

The present invention still further provides the use of catalystregeneration of a moving bed reactor for the catalytic cracking of anolefin-containing feedstock which is selective towards lighter olefins,the catalyst regeneration being employed to average out propylene purityto higher values observed in a fixed bed reaction during an initialperiod, typically from 10 to 40 hours, of the olefin-cracking process.

Preferably, the catalyst regeneration is also employed to average outthe high ethylene yield during the initial period and the low ethyleneyield during the final period observed in a fixed bed reactor.

The feedstock having at least C₄+ hydrocarbons may be an effluent from afluidised bed catalytic cracking (FCC) unit in an oil refinery.

The present invention provides a solution to the problem of loss ofactivity of the catalyst by the addition of the steps of removingdeactivated catalyst from, and feeding reactivated catalyst into, thecatalytic conversion zone which compensates for loss of activity withoutraising the reaction temperature, in particular, by using a moving bedreactor in which the catalyst circulates between a catalytic conversionzone and a catalyst regeneration zone. A moving bed reactor/regenerationcombination still provides the possibility to operate the reactionsection and regeneration section independently as they are physicallyisolated by means of lock hoppers and valves between the differentsections. Each section can thus operate at its own optimal conditionsand moreover the regeneration section can be temporarily shut down whilethe reaction section continues to operate.

When employing a moving bed reactor in which intermittently catalyst iswithdrawn and regenerated and consequently re-injected into thecatalytic reaction zone, the catalytic performance of the catalyst inthe catalytic reaction zone can be maintained constant. This will resultin a constant product distribution over time. Moreover, the less desiredproduct formation, observed at the start of the catalytic cycle in fixedbed reactors, can thus be moderated because the catalytic performance ina moving bed reactor is an average of the catalytic performance observedin fixed bed reactors.

The present invention is predicated on the discovery by the inventorthat in order to achieve a propylene purity i.e. a proportion ofpropylene in the total C₃ content of the effluent, of at least 94 wt %,and preferably also to achieve an ethylene yield on an olefin basisbelow 10 wt %, then the use of a moving bed reactor with catalystregeneration enables these average values to be achieved on a continuousbasis, more particularly by regulating the catalyst regenerationaccording to the desired propylene purity, and optionally depending onthe ethylene content, which is dependent upon the particular commercialrequirements for the proportion of ethylene in the effluent, whereby theentire catalyst content of the moving bed reactor is regenerated in aperiod of from 20 to 240 hours. The particular period within which theentire body of catalyst in the moving bed reactor is regenerated dependson a number of factors, including the nature of the particular catalyst,temperature, LHSV, feedstock content, etc. Fundamentally, the catalystregeneration is carried out so that the average values of propylenepurity, and preferably also ethylene yield on an olefin basis, are suchas to enable high purity propylene to be produced, with the averagingessentially overcoming the technical problem of low propylene purity andoptionally high ethylene yield on an olefin basis during the initialperiod of a fixed bed reactor, typically up to the first 10 to 40 e.g.20 or 30 hours, of the olefin cracking process. This overcomes thetechnical problem present in the prior art, in particular inEP-A-0921179, of low propylene purity, and optionally also high ethyleneyield on an olefin basis, reducing the ability of the catalyst toproduce acceptable chemical grade purity propylene, and optionally lowethylene content, over acceptable run times.

The preferred embodiment of the present invention can thus provide aprocess using a catalyst for the production of a catalytic reactoreffluent characterised by a constant composition by utilising a movingbed reactor in which the catalyst circulates between a catalyticconversion zone and a catalyst regeneration zone. The preferredembodiments of the present invention can also provide a process using acatalyst whereby the formation of less desired products over freshcatalyst is tempered to an average acceptable level by utilising amoving bed reactor in which the catalyst circulates between a catalyticconversion zone and a catalyst regeneration zone.

The present invention can thus provide a process wherein olefin-richhydrocarbon streams (products) from refinery and petrochemical plantsare selectively cracked not only into light olefins, but particularlyinto propylene. In one embodiment, the olefin-rich feedstock is passedover an MFI-type crystalline silicate catalyst with a particular Si/Alatomic ratio of either at least 180 attained after asteaming/de-alumination treatment or at least 300 with the catalysthaving been prepared by crystallisation using an organic template andhaving been unsubjected to any subsequent steaming or de-aluminationprocess. In another embodiment, the olefin-rich feedstock is passed overan MEL-type crystalline silicate catalyst, with a particular Si/Alatomic ratio and which has been steamed for example at a temperature ofat least 300° C. for a period of at least 1 hour with a water partialpressure of at least 10 kPa. The feedstock may be passed over thecatalyst at a temperature ranging between 500 to 600° C., an olefinpartial pressure of from 0.1 to 2 bars and an LHSV of from 5 to 30 h⁻¹.This can yield at least 30 to 50% propylene based on the olefin contentin the feedstock, with a selectivity to propylene for the C₃ speciespropylene and propane (i.e. a C₃ ⁻/C₃s ratio) of at least 92% by weight.

In this specification, the term “silicon/aluminium atomic ratio” isintended to mean the Si/Al atomic ratio of the overall material, whichmay be determined by chemical analysis. In particular, for crystallinesilicate materials, the stated Si/Al ratios apply not just to the Si/Alframework of the crystalline silicate but rather to the whole material.

The feedstock may be fed either undiluted or diluted with an inert gassuch as nitrogen. In the latter case, the absolute pressure of thefeedstock constitutes the partial pressure of the hydrocarbon feedstockin the inert gas.

In accordance with the present invention, cracking of olefins isperformed in the sense that olefins in a hydrocarbon stream are crackedinto lighter olefins and selectively into propylene. The feedstock andeffluent preferably have substantially the same olefin content byweight. Typically, the olefin content of the effluent is within ±15 wt%, more preferably ±10 wt %, of the olefin content of the feedstock. Thefeedstock may comprise any kind of olefin-containing hydrocarbon stream.The feedstock may typically comprise from 10 to 100 wt % olefins andfurthermore may be fed undiluted or diluted by a diluent, the diluentoptionally including a non-olefinic hydrocarbon. In particular, theolefin-containing feedstock may be a hydrocarbon mixture containingnormal and branched olefins in the carbon range C₄ to C₁₀, morepreferably in the carbon range C₄ to C₆, optionally in a mixture withnormal and branched paraffins and/or aromatics in the carbon range C₄ toC₁₀. Typically, the olefin-containing stream has a boiling point of fromaround −15 to around 180° C.

In particularly preferred embodiments of the present invention, thehydrocarbon feedstocks comprise C₄ mixtures from refineries and steamcracking units. Such steam cracking units crack a wide variety offeedstocks, including ethane, propane, butane, naphtha, gas oil, fueloil, etc. Most particularly, the hydrocarbon feedstock may comprises aC₄ cut from a fluidised-bed catalytic cracking (FCC) unit in a crude oilrefinery which is employed for converting heavy oil into gasoline andlighter products. Typically, such a C₄ cut from an FCC unit comprisesaround 50 wt % olefin. Alternatively, the hydrocarbon feedstock maycomprise a C₄ cut from a unit within a crude oil refinery for producingmethyl tert-butyl ether (MTBE) which is prepared from methanol andisobutene. Again, such a C₄ cut from the MTBE unit typically comprisesaround 50 wt % olefin. These C₄ cuts are fractionated at the outlet ofthe respective FCC or MTBE unit. The hydrocarbon feedstock may yetfurther comprise a C₄ cut from a naphtha steam-cracking unit of apetrochemical plant in which naphtha, comprising C₅ to C₉ species havinga boiling point range of from about 15 to 180° C., is steam cracked toproduce, inter alia, a C₄ cut. Such a C₄ cut typically comprises, byweight, 40 to 50% 1,3-butadiene, around 25% isobutylene, around 15%butene (in the form of but-1-ene and/or but-2-ene) and around 10%n-butane and/or isobutane. The olefin-containing hydrocarbon feedstockmay also comprise a C₄ cut from a steam cracking unit after butadieneextraction (Raffinate 1), or after butadiene hydrogenation.

In accordance with the present invention, the catalyst for the crackingof the olefins comprises a crystalline silicate of the MFI family whichmay be a zeolite, a silicalite or any other silicate in that family orthe MEL family which may be a zeolite or any other silicate in thatfamily. Examples of MFI silicates are ZSM-5 and silicalite. An exampleof an MEL zeolite is ZSM-11 which is known in the art. Other examplesare Boralite D, and silicalite-2 as described by the InternationalZeolite Association (Atlas of zeolite structure types, 1987,Butterworths).

The preferred crystalline silicates have pores or channels defined byten oxygen rings and a high silicon/aluminium atomic ratio.

Crystalline silicates are microporous crystalline inorganic polymersbased on a framework of XO₄ tetrahydra linked to each other by sharingof oxygen ions, where X may be trivalent (e.g. Al, B, . . . ) ortetravalent (e.g. Ge, Si, . . . ). The crystal structure of acrystalline silicate is defined by the specific order in which a networkof tetrahedral units are linked together. The size of the crystallinesilicate pore openings is determined by the number of tetrahedral units,or, alternatively, oxygen atoms, required to form the pores and thenature of the cations that are present in the pores. They possess aunique combination of the following properties: high internal surfacearea; uniform pores with one or more discrete sizes; ionexchangeability; good thermal stability; and ability to adsorb organiccompounds. Since the pores of these crystalline silicates are similar insize to many organic molecules of practical interest, they control theingress and egress of reactants and products, resulting in particularselectivity in catalytic reactions. Crystalline silicates with the MFIstructure possess a bi-directional intersecting pore system with thefollowing pore diameters: a straight channel along [10]: 0.53-0.56 nmand a sinusoidal channel along [100]: 0.51-0.55nm. Crystalline silicateswith the MEL structure possess a bi-directional intersecting straightpore system with straight channels along [100] having pore diameters of0.53-0.54 nm.

The crystalline silicate catalyst has structural and chemical propertiesand is employed under particular reaction conditions whereby thecatalytic cracking readily proceeds. Different reaction pathways canoccur on the catalyst. Under the process conditions, having an inlettemperature of around 500 to 600° C., preferably from 520 to 600° C.,yet more preferably 540 to 580° C., and an olefin partial pressure offrom 0.1 to 2 bars, most preferably around atmospheric pressure, theshift of the double bond of an olefin in the feedstock is readilyachieved, leading to double bond isomerisation. Furthermore, suchisomerisation tends to reach a thermodynamic equilibrium. Propylene canbe, for example, directly produced by the catalytic cracking of hexeneor a heavier olefinic feedstock. Olefinic catalytic cracking may beunderstood to comprise a process yielding shorter molecules via bondbreakage.

With such high silicon/aluminum ratio in the crystalline silicatecatalyst, a stable olefin conversion can be achieved with a highpropylene yield on an olefin basis of from 30 to 50% whatever the originand composition of the olefinic feedstock. Such high ratios reduce theacidity of the catalyst, thereby increasing the stability of thecatalyst.

The MFI catalyst having a high silicon/aluminum atomic ratio for use inthe catalytic cracking process of the present invention may bemanufactured by removing aluminum from a commercially availablecrystalline silicate. A typical commercially available silicalite has asilicon/aluminum atomic ratio of around 120. The commercially availableMFI crystalline silicate may be modified by a steaming process whichreduces the tetrahedral aluminum in the crystalline silicate frameworkand converts the aluminum atoms into octahedral aluminum in the form ofamorphous alumina. Although in the steaming step aluminum atoms arechemically removed from the crystalline silicate framework structure toform alumina particles, those particles cause partial obstruction of thepores or channels in the framework. This inhibits the olefinic crackingprocesses of the present invention. Accordingly, following the steamingstep, the crystalline silicate is subjected to an extraction stepwherein amorphous alumina is removed from the pores and the microporevolume is, at least partially, recovered. The physical removal, by aleaching step, of the amorphous alumina from the pores by the formationof a water-soluble aluminum complex yields the overall effect ofde-alumination of the MFI crystalline silicate. In this way by removingaluminum from the MFI crystalline silicate framework and then removingalumina formed therefrom from the pores, the process aims at achieving asubstantially homogeneous de-alumination throughout the whole poresurfaces of the catalyst. This reduces the acidity of the catalyst, andthereby reduces the occurrence of hydrogen transfer reactions in thecracking process. The reduction of acidity ideally occurs substantiallyhomogeneously throughout the pores defined in the crystalline silicateframework. This is because in the olefin-cracking process hydrocarbonspecies can enter deeply into the pores. Accordingly, the reduction ofacidity and thus the reduction in hydrogen transfer reactions whichwould reduce the stability of the MFI catalyst are pursued throughoutthe whole pore structure in the framework. The frameworksilicon/aluminum ratio may be increased by this process to a value of atleast about 180, preferably from about 180 to 1000, more preferably atleast 200, yet more preferably at least 300, and most preferably around480.

The MEL or MFI crystalline silicate catalyst may be mixed with a binder,preferably an inorganic binder, and shaped to a desired shape, e.g.extruded pellets. The binder is selected so as to be resistant to thetemperature and other conditions employed in the catalyst manufacturingprocess and in the subsequent catalytic cracking process for theolefins. The binder is an inorganic material selected from clays,silica, metal oxides such as ZrO₂ and/or metals, or gels includingmixtures of silica and metal oxides. The binder is preferablyalumina-free. Although aluminium in certain chemical compounds as inAlPO₄'s may be used as the latter are quite inert and not acidic innature. If the binder which is used in conjunction with the crystallinesilicate is itself catalytically active, this may alter the conversionand/or the selectivity of the catalyst. Inactive materials for thebinder may suitably serve as diluents to control the amount ofconversion so that products can be obtained economically and orderlywithout employing other means for controlling the reaction rate. It isdesirable to provide a catalyst having a good crush strength. This isbecause in commercial use, it is desirable to prevent the catalyst frombreaking down into powder-like materials. Such clay or oxide bindershave been employed normally only for the purpose of improving the crushstrength of the catalyst. A particularly preferred binder for thecatalyst of the present invention comprises silica.

The relative proportions of the finely divided crystalline silicatematerial and the inorganic oxide matrix of the binder can vary widely.Typically, the binder content ranges from 5 to 95% by weight, moretypically from 20 to 50% by weight, based on the weight of the compositecatalyst. Such a mixture of crystalline silicate and an inorganic oxidebinder is referred to as a formulated crystalline silicate.

In mixing the catalyst with a binder, the catalyst may be formulatedinto pellets, spheres, extruded into other shapes, or formed into aspray-dried powder. For practising the present invention it is preferredthat the formulated catalyst has a very symmetrical shape like inspheres and pellets or extrudates having equal height and wideness. Itis important that the settling velocity of the catalyst particles in agas stream is the same for all orientations relative to the gas streamdirection.

In the catalytic cracking process, the process conditions are selectedin order to provide high selectivity towards propylene, a stable olefinconversion over time, and a stable olefinic product distribution in theeffluent. Such objectives are favoured by the use of a low acid densityin the catalyst (i.e. a high Si/Al atomic ratio) in conjunction with alow pressure, a high inlet temperature and a short contact time, all ofwhich process parameters are interrelated and provide an overallcumulative effect (e.g. a higher pressure may be offset or compensatedby a yet higher inlet temperature). The process conditions are selectedto disfavour hydrogen transfer reactions leading to the formation ofparaffins, aromatics and coke precursors. The process operatingconditions thus employ a high space velocity, a low pressure and a highreaction temperature. The LHSV ranges from 5 to 30 h⁻¹, preferably from10 to 30 h⁻¹. The olefin partial pressure ranges from 0.1 to 2 bars,preferably from 0.5 to 1.5 bars. A particularly preferred olefin partialpressure is atmospheric pressure (i.e. 1 bar). The hydrocarbonfeedstocks are preferably fed at a total inlet pressure sufficient toconvey the feedstocks through the reactor. The hydrocarbon feedstocksmay be fed undiluted or diluted in an inert gas, e.g. nitrogen.Preferably, the total absolute pressure in the reactor ranges from 0.5to 10 bars. The use of a low olefin partial pressure, for exampleatmospheric pressure, tends to lower the incidence of hydrogen transferreactions in the cracking process, which in turn reduces the potentialfor coke formation which tends to reduce catalyst stability. Thecracking of the olefins is preferably performed at an inlet temperatureof the feedstock of from 500 to 600° C., more preferably from 520 to600° C., yet more preferably from 540 to 590° C., typically around 560°C. to 585° C.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic process scheme in accordance with one embodimentof the present invention for processing refinery and/or petrochemicalfeedstocks, the process scheme incorporating a process for selectivelycatalytically cracking olefins into lighter olefins over a crystallinesilicate catalyst, and incorporating catalyst regeneration;

FIG. 2 shows a schematic process scheme in accordance with a secondembodiment of the present invention for processing refinery and/orpetrochemical feedstocks, the process scheme incorporating a process forselectively catalytic cracking olefins into lighter olefins over acrystalline silicate catalyst and catalyst regeneration;

FIG. 3 shows a schematic process scheme in accordance with a thirdembodiment of the present invention for processing refinery and/orpetrochemical feedstocks, the process scheme incorporating a process forselectively catalytically cracking olefins into lighter olefins over acrystalline silicate catalyst and catalyst regeneration;

FIG. 4 shows the relationship between the olefin content of an effluentand time for one example of a catalytic cracking process; and

FIG. 5 shows the relationship between olefin content and time for asecond example of a catalytic cracking process.

FIG. 1 provides a schematic illustration of a configuration forpractising the process of the present invention. The description is notintended to exclude certain modifications and in order to simplify thedrawing shut-off valves, solid flow controlling valves, pumps, pipingand other conventional equipment readily known by the person skilled inthe art are not shown.

The fresh olefin-containing feed to be catalytically cracked andpreferably combined with recycle feed, and optionally a diluting gaslike hydrogen, steam or any other inert gas, are sent through line 1 toa feed-effluent heat exchanger 2 and further through line 3 to a heater4 to raise the temperature of the mixture to the desired reactiontemperature. Through line 5 the hot mixture is sent into a radial-flowreactor 10. The reactor 10 contains an annulus of dense phase catalyst.The feed mixture may be injected in the centre of the annulus and mayleave the catalyst external to the catalyst bed annulus. Optionally, thefeed mixture may be injected in the catalyst bed external to the bedannulus and may leave the catalyst bed annulus in the centre of theannulus. The reaction products leave the reaction section through line19 via the feed-effluent heat exchanger 2 to the fractionation section(not shown). In the fractionation section the different reactionproducts are concentrated. Unconverted feed or a produced butene-rich C4fraction may be recycled together with fresh feed to the reactionsection through line 1.

In accordance with the catalyst regeneration in the moving bed reactorin the present invention, the catalyst travels down under gravitythrough the catalyst bed annulus and is continuously or intermittentlywithdrawn through line 20 into a lock hopper 21 where the catalyst ispurged with nitrogen in order to remove hydrocarbon vapours from thecatalyst. In the lock hopper the pressure is equalised to that of a liftengager 22. The catalyst is lifted from the lift engager 22 by means ofa lift gas coming through line 23 to a lift dis-engager 30 through acatalyst lift line 24. The gaseous lift gas may be hydrogen, nitrogen,methane, steam or even diluted oxygen in nitrogen. The flow rate of thelift gas is sufficient to surpass the settling velocity of the catalystparticles in order to transfer the catalyst through the lift line 24 toa lift dis-engager 30. In the lift dis-engager 30, the catalyst isseparated from the lift gases through line 31 and the pressure isequalised to the pressure of a catalyst regeneration vessel 40. The liftgases may be recycled or sent to other purposes. The catalyst is fedfrom the lift dis-engager 30 through line 32 to the regeneration vessel40.

In the regeneration vessel 40 the carbonaceous material laid down on thecatalyst is burned off by means of oxygen, to form carbon dioxide. Theregeneration vessel 40 may consist of a cylindrical moving bed ofcatalyst travelling down by gravity. Alternatively, it may also consistof a radial-flow type catalyst bed. The oxidising gases are injected inthe centre of the catalyst bed annulus or from the exterior of theannulus. Fresh air is provided through line 41, mixed with recycle gascoming through line 48 and compressed by means of a compressor 42 intoline 43. The oxygen containing mixture goes from line 43 into theregeneration vessel 40. The combustion gases leave the regenerationvessel through line 44 and goes to a vessel 45. The combustion gases arecooled down or heat exchanged and eventually dried. Water is drained offthrough line 46. Uncondensed gases are partially purged out through line47, and the remaining may be recycled and mixed with fresh air throughline 41.

To control the combustion of the carbonaceous material on the catalystthe oxygen should be present at relatively low concentrations. The ratioof recycle gas to fresh air is generally high. The volume percent ofoxygen in the oxidising gas is typically from 0.2 to 2, preferably about0.6. Other compounds may be present in the oxidising gas, such as carbondioxide, nitrogen and optionally carbon monoxide.

During the regeneration the catalyst travels down under gravity and thecarbonaceous material is progressively burned off. It may be desirableto use higher concentrations of oxygen towards the end of theregeneration vessel 40. A second inlet of oxygen containing gas may beinjected into the regeneration vessel 40 more to the lower parts of thecatalyst bed where carbonaceous material is already burned off to agreat extent. As is known, regeneration with oxygen is exothermic andcare should be taken not to exceed the temperature at which the catalystis damaged. It is preferred not to surpass 600° C. in the catalyst bed.The regeneration is generally started at about 450° C. Therefore theoxygen containing gas may be heated up before entering the regenerationvessel 40. The second oxygen containing stream which may be injectedinto the regeneration vessel may be heated up to a higher temperature tofinish better the burn off of carbonaceous materials laid down on thecatalyst. The value percent of oxygen in the second oxygen-containingstream is typically from 2 to 100, preferably from 5 to 21. Othercompounds may be present in the oxidising gas, such as carbon dioxide,nitrogen and optionally carbon monoxide.

The catalyst flows through line 50 to a lock hopper 51. Optionally, theregeneration may be finished here by purging first the hopper 51 withpure air at the highest allowable temperature for the catalyst, followedby a nitrogen purge in order to remove any remaining oxygen. Thecatalyst further flows through line 52 to a lift engager 53. By means ofa lift gas, coming through line 54, the catalyst is sent to a catalystcollector hopper 61 located above the reactor 10 through a catalysttransfer line 60. The catalyst is separated from the lift gases throughline 62. These lift gases may be sent to other purposes or may berecycled and used again as lift gas. The pressure in the catalystcollector hopper 61 is equalised to the reactor pressure. Theregenerated catalyst in the collector hopper 61 flows through line 63into the reactor vessel 10. New fresh catalyst may be added into thecatalyst collector hopper 61 through line 64, while used catalyst can bewithdrawn from the regeneration system through line 65.

FIG. 2 shows an alternative embodiment for practising the presentinvention. As the cracking of long-chain olefins into lighter olefins isan endothermic reaction, it may be desired to reheat the reactionmixture. FIG. 2 shows the alternative embodiment with two moving bedreactors 10, 15 in series for the olefin-cracking process. The reactoreffluent of the first radial-flow reactor 10 leaves the reactor throughline 11 and is sent to a reheater 12. The mixture is sent through line13 into the second reactor 15. The second reactor 15 can be locatedbelow the first rector 10 as illustrated or optionally the secondreactor 15 is parallel to the first reactor 10. In the latter case,there is provided a catalyst lift transfer line (not shown) between thefirst and the second reactors 10, 15. The rest of the process scheme isas explained above for FIG. 1. As the catalyst becomes less active whenmoving down through the moving bed, it may be desired to increase thecontact time of the reaction mixture with the catalyst in the secondreactor. This can easily be done by increasing the thickness of thecatalyst bed annulus.

A still other embodiment for practising the present invention is shownin FIG. 3. As the reactors are not very large, it can be advantageous toplace the regeneration vessel 40 on top of the first reactor 10 (or thesingle reactor 10 as shown in FIG. 1). This implies one fewer catalysttransfer line which will reduce the attrition of the catalyst due to thetransport step.

The present invention will now be described with reference to thefollowing non-limiting examples.

EXAMPLE 1

A feedstock having the feed composition shown in Table 1, consisting ofa 50/50 wt % mixture of C₄s and LCCS produced on an FCC unit wassubjected to olefin catalytic cracking in a fixed bed reactor (not inaccordance with the invention) comprising a crystalline silicatecatalyst of the MFI-type (as generally disclosed in EP-A-0921179) havinga silicon/aluminium atomic ratio of at least 270 at an inlet temperatureof 585° C., a liquid hourly space velocity (LHSV) of 20 h⁻¹ and anoutlet pressure of 0.5 bara. The composition of the effluent over timewas measured to determine the propylene (C₃—) content, the ethylene(C₂—) content, the isobutene (i-C4-) content and the propylene purityand the results are shown in FIG. 4. The reactor is loaded with 5 litresof catalyst and the reactor operates in an adiabatical mode.

From FIG. 4 it may be seen that the propylene content, i.e. the yield onan olefin basis towards propylene of the olefin-cracking process, isinitially slightly greater than or about 35 wt % up to a period ofaround 35 hours, after which the propylene content rapidly decreases toa value of as low as about 18 wt % after a period of about 75 hours.This shows that the activity of the catalyst towards the production ofpropylene in the olefin-cracking process reduces over time, specificallyfor runs greater than around 35 hours. In addition, for shorter reactiontimes on stream, there are problems in that the ethylene content on anolefin basis of the effluent is initially high, starting from greaterthan 10 wt % and being greater than 5 wt % up to 40 hours on stream, andalso the propylene purity (i.e. the ratio of propylene to total C₃content) is initially low and increases to a value greater than 94 wt %only after a period of around 10 hours on stream.

Table 2 shows values of the propylene content, ethylene content,isobutene content and propylene purity after 4 specific times on stream,up to about 35 hours on stream during which the propylene yield is quiteconstant.

In accordance with the process of the present invention, by providing amoving bed reactor with continuous catalyst regeneration, the fourdiscrete yields in the effluent are substantially averaged to yield theaverage values also specified in Table 2. It may thus be seen that byusing a moving bed reactor in conjunction with continuous catalystregeneration, the composition of the effluent may be made more constant,in particular the propylene content and purity. Moreover, the formationof less desired products in the effluent, such as ethylene, whichrequires a relatively difficult fractionation process to be separatedfrom the desired propylene, reduced continuously to an averageacceptable level as compared to the initial level in the case of a fixedbed.

EXAMPLE 2

In accordance with this Example, the same feed having a typicalcomposition illustrated in Table 1 was fed over the same catalyst as inExample 1 and at the same inlet temperature and outlet pressure, but ata lower LHSV of 10 h⁻¹. The relationship between the olefin content andtime on stream is illustrated in FIG. 5. Table 3 shows the variationbetween the propylene, ethylene and isobutene contents with time,together with the propylene purity variation with time.

As for Example 1, for Example 2 it may be seen that the use of a movingbed reactor together with catalyst regeneration provides a substantiallyaverage value for the composition of the effluent which tends to providean improved average value for the ethylene content and an improvedaverage value for the propylene purity. TABLE 1 FEED COMPOSITION [WT %]C nr F O N A Total Cumulative 1 0.00 2 0.00 0.00 3 0.02 0.28 0.31 0.31 417.40 25.74 43.14 43.45 5 6.03 6.90 0.09 13.03 56.48 6 8.84 5.54 1.580.52 16.48 72.96 7 3.98 3.13 2.39 3.00 12.49 85.45 8 3.23 1.12 2.06 3.8810.28 95.74 9 1.49 0.26 0.11 1.86 3.72 99.46 10 0.11 0.00 0.00 0.40 0.5199.97 11 0.03 0.03 100.00 Total 41.10 42.97 6.23 9.70 100.00

TABLE 2 EXAMPLE 1 TOS [h] 4.17 19.48 34.57 AVERAGE C3-[wt %] 36.27 34.9535.14 35.45 C2-[wt %] 10.03 7.26 6.35 7.88 i-C4-[wt %] 13.56 18.48 22.6618.23 c3-purity [wt %] 93.44 94.94 95.45 94.61

TABLE 3 EXAMPLE 2 TOS[h] 2.47 4.92 10.18 17.83 28.37 41.10 AVERAGEC3-[wt %] 32.93 33.71 33.61 33.49 35.53 34.09 33.89 C2-[wt %] 10.5910.71 9.93 9.11 9.03 7.54 9.48 i-C4-[wt %] 11.51 11.96 12.31 12.93 14.3415.47 13.09 c3-pur- 88.38 90.42 91.79 93.08 93.81 94.97 92.08 ity[wt %]

1. A process for cracking an olefin-containing hydrocarbon feedstockwhich is selective towards light olefins in the effluent, the processcomprising passing a hydrocarbon feedstock containing one or moreolefins through a moving bed reactor containing a crystalline silicatecatalyst selected from an MFI-type crystalline silicate having asilicon/aluminum atomic ratio of at least 180 and an MEL-typecrystalline silicate having a silicon/aluminum atomic ratio from 150 to800 which has been subjected to steaming step, at an inlet temperatureof from 500 to 600° C., at an olefin partial pressure of from 0.1 to 2bars and the feedstock being passed over the catalyst at an LHSV of from5 to 30 h⁻¹ to produce an effluent containing propylene and with anolefin content of lower molecular weight than the olefin content of thefeedstock with concomittant deactivation of said catalyst, removing afirst fraction of the deactivated catalyst from the moving bed reactorand transferring said deactivated catalyst to a regenerator,regenerating said deactivated catalyst in said regenerator to produce asecond fraction of regenerated catalyst and recycling said regeneratedcatalyst to the moving bed reactor, continuing the transfer ofdeactivated catalyst and the recycle of regenerated catalyst whilecarrying out the cracking of the olefin-containing hydrocarbonfeedstock, the catalyst regeneration and recycle rate being controlledto maintain the propylene purity at a relatively constant valuecorresponding to the average value observed in a fixed bed reactor usingthe same feedstock, catalyst and cracking conditions.
 2. The process ofclaim 1 wherein the catalyst regeneration and recycle rate is controlledto provide an ethylene yield in the effluent on an olefin basis which isless than 10 wt %.
 3. The process of claim 1, wherein the effluent has apropylene purity of at least 94 wt % propylene based upon the total C₃content of the effluent.
 4. The process of claim 1, wherein the olefincontent of the effluent is within ±15 wt % of the olefin content of thefeed stock.
 5. The process of claim 1, wherein said first fraction ofthe catalyst is intermittently removed from said moving bed reactor. 6.The process of claim 5, wherein said second fraction of the regeneratedcatalyst is intermittently supplied from said regenerator to said movingbed reactor.
 7. The process of claim 1, wherein said catalyst isregenerated in said regenerator by supplying an oxidizing gas containingoxygen in amount within the range of 0.2 to 2 volume percent of saidoxidizing gas.
 8. The method of claim 1, wherein the regeneration ofcatalyst in said regenerator involves a supply of an initialoxygen-containing gas to the regenerator and a supply of a secondoxygen-containing gas to the regenerator at a point downstream of theintroduction of said initial oxygen-containing gas, said secondoxygen-containing gas having a higher oxygen content than said initialoxygen-containing gas.
 9. The process of claim 8, wherein said secondoxygen-containing gas contains from 5 to 21 volume percent oxygen. 10.The process of claim 1, wherein said moving bed reactor comprises afirst stage reactor and a second stage reactor connected in series withsaid first stage reactor, wherein the effluent from the first stagereactor is heated and then supplied to the inlet of said second stagereactor.
 11. The process of claim 10, wherein the contact time of thereaction mixture with the catalyst in the second reactor is greater thanthe contact time of the reaction mixture with the catalyst in the firststage reactor.
 12. A process for cracking an olefin-containinghydrocarbon feedstock which is selective towards light olefins in theeffluent, the process comprising passing a hydrocarbon feedstockcontaining one or more olefins through a moving bed reactor containing acrystalline silicate catalyst selected from an MFI-type crystallinesilicate having a silicon/aluminum atomic ratio of at least 180 and anMEL-type crystalline silicate having a silicon/aluminum atomic ratiofrom 150 to 800 which has been subjected to steaming step, at an inlettemperature of from 500 to 600° C., at an olefin partial pressure offrom 0.1 to 2 bars and the feedstock being passed over the catalyst atan LHSV of from 5 to 30 h⁻¹ to produce an effluent with an olefincontent of lower molecular weight than that of the feedstock withconcomittant deactivation of said catalyst, removing a first fraction ofthe deactivated catalyst from the moving bed reactor and transferringsaid deactivated catalyst to a regenerator, regenerating saiddeactivated catalyst in said regenerator to produce a second fraction ofregenerated catalyst and recycling said regenerated catalyst to themoving bed reactor, continuing the transfer of deactivated catalyst andthe recycle of regenerated catalyst while carrying out the cracking ofthe olefin-containing hydrocarbon feedstock, the catalyst regenerationand recycle rate being controlled to provide that all of the catalyst inthe moving bed reactor is regenerated in a period of from 20 to 240hours.
 13. The process of claim 12 wherein the propylene purity ismaintained at a relative constant value corresponding to the averagevalue obtained in a fixed bed reactor using the same feedstock, catalystand cracking conditions.
 14. The process of claim 12 where theregeneration and recycle rate is controlled to proceed on ethylene yieldon an olefin basis which is less than 10 wt %.
 15. The process of claim12, wherein the propylene yield of said process is within the range of30 to 50 wt % propylene with a selectivity to propylene of at least 92wt % of the total amount of propylene and propane in the effluent. 16.The process of claim 15, wherein the olefin content of the effluent iswithin the range of ±10 wt % of the olefin content of the feed stock.17. The method of claim 12, wherein the regeneration of catalyst in saidregenerator involves a supply of an initial oxygen-containing gas to theregenerator and a supply of a second oxygen-containing gas to theregenerator at a point downstream of the introduction of said initialoxygen-containing gas, said second oxygen-containing gas having a higheroxygen content than said initial oxygen-containing gas.
 18. The processof claim 17, wherein said second oxygen-containing gas contains from 5to 21 volume percent oxygen.
 19. The process of claim 12, wherein saidmoving bed reactor comprises a first stage reactor and a second stagereactor connected in series with said first stage reactor, wherein theeffluent from the first stage reactor is heated and then supplied to theinlet of said second stage reactor.
 20. The process of claim 17, whereinthe contact time of the reaction mixture with the catalyst in the secondstage reactor is greater than the contact time of the reaction mixturewith the catalyst in the first stage reactor.
 21. A process for crackingan olefin-containing hydrocarbon feedstock which is selective towardslight olefins in the effluent, the process comprising passing ahydrocarbon feedstock containing one or more olefins through a movingbed reactor containing a crystalline silicate catalyst to produce aneffluent with an olefin content of lower molecular weight than that ofthe feedstock with concomittant deactivation of said catalyst, removinga deactivated catalyst from the moving bed reactor and regenerating saiddeactivated catalyst to produce a regenerated catalyst which is recycledto said moving bed reactor, continuing the removal of deactivatedcatalyst and the recycle of regenerated catalyst while carrying out thecracking of the olefin-containing hydrocarbon feedstock, the catalystregeneration and recycle rate being controlled to provide a propylenecontent in the effluent that is maintained relatively constant at avalue corresponding to the initial value observed in the effluent from afixed bed reactor during an initial period of an olefin cracking processusing the same feedstock, catalyst and cracking conditions.
 22. Theprocess of claim 21, wherein the initial period of the fixed bedreaction is the first 10 to 40 hours of said reaction.
 23. The processaccording to claim 21, wherein said moving bed reactor comprises a firststage reactor and a second stage reactor connected in series with saidfirst stage reactor, wherein the effluent from the first stage reactoris heated and then supplied to the inlet of said second stage reactor.24. The process of claim 23, wherein the contact time of the reactionmixture with the catalyst in the second reactor is greater than thecontact time of the reaction mixture with the catalyst in the firststage reactor.
 25. The process of claim 21, wherein fresh makeupcatalyst is supplied to said moving bed reactor along with saidregenerated catalyst recycled to said moving bed reactor.